The present invention relates to a single lens reflex camera with an automatic exposure control device.
In a camera of this type, the diaphragm is first stopped down upon actuation of a shutter release operation and then the light passing through the stopped-down diaphragm aperture is measured and stored. Subsequent to storing a signal indicative of the measured light, a movable mirror is retracted from its viewing position along the optical path of the camera. Exposure is then effectuated for a duration depending on the amount of light measured and stored.
With such a single lens reflex camera, accurate automatic exposure-time control cannot be attained, unless the light passing through the diaphragm aperture is measured and stored after movement of the diaphragm has been stabilized following the stopping-down operation. The light which is measured and stored is that transmitted through or reflected off the mirror at its viewing position. For this reason, it is not possible to simultaneously initiate movement of the movable mirror from its viewing position to its retracted position and the stopping-down of the diaphragm aperture. Such simultaneous initiation which can be obtained with a full aperture TTL light-measuring type camera is represented by the time chart shown in FIG. 1.
With the camera of concern in the present invention, however, movement of the mirror has to be effected after the diaphragm has been completely stopped down. In the previously known systems, the direction in which the focal plane shutter travels upon exposure is generally different from the direction of movement of the mirror to its retracted position, e.g., mirror is retracted upwardly while the shutter curtain travels from left to right. Accordingly, exposure is not effected during the lapse of time from the commencement of the retracting movement of the movable mirror from the picture-taking optical path until termination of such a movement, as represented in the time chart shown in FIG. 2. As shown in FIGS. 1 and 2, movement of the first curtain is controlled to start after arrival of the mirror at the retracted or photographing position. It has long been recognized that with such systems the exposure can only be commenced when the optical path is free of any interruption. Thus, with the prior art type of camera, there is a considerable time lag from the time when the shutter is released until a picture is actually taken. Such a long time lag occasionally results in production of a picture far from the photographer's expectation.
In this type of camera, light can be measured in two different ways, i.e., with the aperture at a fully-open position or at a stopped-down position, as previously noted.
i. Fully-open aperture light measurement (FIG. 1)
Light is measured and stored with the diaphragm aperture fully open. In utilizing this type of measurement, it is possible to drive the mirror to the photographing position simultaneously with the stopping-down operation because the storage of the light measuring output has been already completed.
ii. Stopped-down aperture light measurement (FIG. 2)
Light is measured and stored after the stopping-down operation. In utilizing this type of measurement, the mirror has to be maintained at the viewing position until storage of the light measuring output is completed.
The following table shows the advantages and disadvantages for fully-open aperture and stopped-down aperture light measurements.
______________________________________ Fully-open aperture Stopped-down aperture measurement measurement ______________________________________ Advantages Less delay of actual Light is measured exposure from the through the stopped- shutter release down aperture through operation which the film is actually exposed No need for any com- pensation Dis- Light is measured Greater delay of advantages through the fully- actual exposure from open aperture, while the shutter release the film is exposed operation through the stopped- down aperture Compensation based on the difference be- tween the fully-open aperture and the preset aperture is necessary for con- trolling the exposure time ______________________________________